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    <title>Document</title>
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    <script>

        // 函数的默认值

        // function sum (a , b) {
        //     // a = a || 0 ;
        //     // b = b || 0 ;
        //     return a + b ;
        // }

        // sum() ;   // NaN


        // function sum(a = 0 , b = 0) {
        //     var a = 0 ;
        //     var b = 0 ;
        //     return a + b ;
        // }

        // console.log(sum(1,2));


        const sum = (a = 0 , b = 0) => a + b ;
        console.log(sum(1,3));

    </script>
    
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